Eraser tool 'jammed'and Interface discombobulated

[rich2005]

You have the alpha-lock enabled for the active layer. The icon  top right of the layers dock toggles on/off  Turn it off.

   

Yet another reason why I dislike the default black theme, layer indications not that obvious

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Reset the docks
In Gimp Edit -> Preferences -> Interface -> Window Management and click on Reset Saved Window Positions to Default Values Restart Gimp https://i.imgur.com/aGM8ooc.jpg